1. Technical Field
The present invention relates to an image forming apparatus for forming images on a print medium fed along a feed path by ejecting ink droplets from an inkjet head(s) onto the print medium.
2. Background Arts
In an image forming apparatus for forming images on a print medium fed along a feed path by ejecting ink droplets from nozzles of inkjet heads onto the print medium, an ink chamber communicating with the nozzles is provided in each of the inkjet heads. To eject the ink droplets from the nozzles, a volume of the ink chamber is changed (increased/decreased: the ink chamber is inflated and deflated) by a drive signal.
While feeding the print medium toward a print position just beneath the ink heads, airflow is generated from an upstream to a downstream along the feed path. In addition, in a case where the print medium is suctioned onto a feed belt by a negative pressure for high-speed feeding, other airflow is also generated beneath the inkjet heads by the negative pressure.
According to such a non-contacting printing method by ejecting ink droplets from nozzles onto a print medium, the ink droplets are drifted downstream along the feed direction by such airflows (hereinafter, referred as feed airflow). Therefore, the ink droplets are deviated from its desired trajectory and landed onto positions other than their target positions. As a result, image degradation may occur due to irregularity of print density (e.g. occurrence of white/black lines, coloration changes of color images and so on).
A Japanese Patent Application Laid-Open No. 2010-173178 (Patent Document 1) discloses a printer (a droplet ejector) that may address the above problems. In the printer, while feeding a print medium, relatively to inkjet heads provided with nozzles, along a direction perpendicular to an alignment direction of the nozzles and ejecting ink droplets, an ejection speed of an ink droplet is made higher as a volume of the ink droplet becomes smaller. According to this ejection speed control, ink dot displacements caused by the feed airflow are restricted.